Dynamics of Hot-Potato Routing in IP Networks

1
Dynamics of Hot-Potato Routing in IP Networks

• Renata Teixeira
• (UC San Diego)
• http//www-cse.ucsd.edu/teixeira
• with
• Aman Shaikh (ATT), Tim Griffin(Intel), and
  Jennifer Rexford(ATT)

SIGMETRICS04 New York, NY
2
Internet Routing Architecture
Web Server
User
Changes in one AS may impact traffic and routing
in other ASes
3
Hot-Potato Routing
New York
San Francisco
ISP network
Dallas
4
Hot-Potato Routing Change
New York
San Francisco
ISP network
Routes to thousands of destinations switch exit
point!!!
Dallas

• Consequences
• Transient forwarding instability
• Traffic shift
• Inter-domain routing changes

5
Approach

• Understanding impact in real networks
• How often hot-potato changes happen?
• How many destinations do they affect?
• What are the convergence delays?
• Main contributions
• Methodology for measuring hot-potato changes
• Characterization on ATTs IP backbone

6
Challenges for Identifying Hot-Potato Changes

• Cannot collect data from all routers
• OSPF flooding gives complete view of topology
• BGP multi-hop sessions to several vantage points
• A single event may cause multiple messages
• Group related routing messages in time
• Router implementation affects message timing
• Controlled experiments of router in the lab
• Many BGP updates caused by external events
• Classify BGP routing changes by possible causes

7
Measurement Methodology
ATT backbone
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Algorithm for Correlating Routing Changes

• Step 1 Process stream of OSPF messages
• Group OSPF messages close in time
• Transform OSPF messages into vantage points
  routing changes
• Step 2 Process stream of BGP updates from
  vantage point
• Group updates close in time
• Classify BGP routing changes by possible OSPF
  cause
• Step 3 Match BGP routing changes to OSPF changes
  in time
• Determine causal relationship

9
Characterization of ATT Network

• Dataset
• BGP updates from 9 routers
• 176 days of data from February to July 2003
• Understanding impact of hot-potato changes
• How often hot-potato changes happen?
• How many destinations do they affect?
• What are the convergence delays?

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Frequency of Hot-Potato Changes
router A router B
Need data from many vantage points and long
duration
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Variation across Routers
dst
dst
NY
NY
SF
SF
9
10
1
1000
B
A
Small changes will make router A switch exit
points to dst
More robust to intradomain routing changes
Important factors - Location relative distance
to egresses - Day which events happen
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Impact of an OSPF Change
router A router B
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Delay for BGP Routing Change

• Steps between OSPF change and BGP update
• OSPF message flooded through the network (t0)
• OSPF updates path cost information
• BGP decision process rerun (timer driven)
• BGP update sent to another router (t)
• First BGP update sent (t1)
• Metrics
• Time for BGP to revisit decision t1 - t0
• Time for BGP update t t0

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BGP Reaction Time
Worst case scenario 0 80 sec to revisit
BGP decision 50 110 sec to send multiple
updates Last prefix may take 3 minutes to
converge!
First BGP update All BGP updates
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Data Plane Convergence
10
R2
R1
111
10
100
E2
E1
dst
Disastrous for interactive applications (VoIP,
gaming, web)
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Conclusion

• Measured impact of hot-potato routing
• Convergence delay (partially fixable)
• Route changes and traffic shifts (fundamental
  property)
• External routing updates
• What to do about it?
• Router vendor event-driven implementation
• Network operator operational practices to avoid
  changes
• Network designer designs that minimize
  sensitivity
• Model of sensitivity to hot-potato disruptions
  (SIGCOMM04)
• Protocol designer looser coupling of routing
  protocols

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Hot-Potato Changes across Prefixes
Cumulative BGP updates
Non hot-potato changes All Hot-potato changes
prefixes
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Algorithm for Correlating Routing Changes
Stream of OSPF messages
Transform OSPF msgs into vantage points
routing changes
Match path cost changes with BGP routing
changes that happened close in time
time
Determine stable routing changes per dst and
classify them according to possible OSPF cause
dst
Stream of BGP updates from vantage point